Positive temperature coefficient (PCT) characteristics refer to properties that a material has conductivity at low temperature, for example, about room temperature, due to low resistance, but drastically increases in electric resistance in a comparatively narrow temperature range as the temperature increases. Thus, a PTC conductive polymer composition having such characteristics is properly used for an electric element, such as a circuit protection element, which changes depending on ambient temperature and current conditions.
A PTC element generally has a structure which includes a resistor of a conductive polymer composition, two electrodes attached to the resistor and connected to a power source, and an electrolyte metal foil on the electrodes.
Normally, a circuit protection element is connected in series with a load in an electric circuit to maintain a low-temperature low-resistance state. However, when exposed to overcurrent or excessive temperature conditions, the element increases in resistance and effectively blocks current flow to the load in the circuit. The element is restored to a low-resistance state upon returning to room-temperature low-current and low-temperature states, while it involves occurrence of NTC characteristics wherein resistance of a resin decreases due to re-agglomeration by Brownian motion and the van der Waals force of a conductive filler when temperature stays at a melting point for a long time or increases to a higher temperature.
Meanwhile, a conventional PTC composition generally includes carbon black or graphite as a conductive polymer. Carbon black and graphite are spherical particles having a minor diameter to major diameter ratio of 80% or higher and readily short-circuit by resin expansion to easily exhibit PTC characteristics, while short-circuited particles may not contact again when bending occurs by external impact (FIG. 1).
Thus, the inventors of the present invention have conducted studies on a PTC element that maintains conductivity even after external impact and discovered that when carbon nanotubes (CNTs) which are thin and long are used for a conductive polymer, the conductive polymer maintains conductivity even after external impact, such as bending, due to good restoring force of the CNTs.
Further, the inventors tried sought a method of using a crystalline polymer having substantial volumetric change as a PTC binder (that is, a first resin) in order to disperse the CNTs tending to agglomerate due to a strong bond between molecules, as compared with carbon black or graphite relatively good in dispersibility and solubility in a general solvent.
However, even though a PTC element is manufactured using CNTs, NTC characteristics occurs upon repeated application of voltage. Thus, as a result of extensive studies, the inventors found out that when a thermosetting resin or a second resin having higher Tm or softening point or greater molecular weight than a first resin is further added, the CNTs and the first resin are fixed to prevent the CNTs from flowing upon repeated change of temperature and to prevent NTC characteristics wherein resistance decreases with increasing temperature.